Analog sensors, for example, thermistors are widely used in various applications to measure temperature. They can be connected via a “single wire interface” as shown in FIG. 1A. Their outputs, however are nonlinear, which requires an algorithm table for interpretation that is usually processed by a microcontroller which is coupled with one of its analog input ports with the thermistor. Thus, as shown in FIG. 1A, the sensor device 110 basically consists of a thermistor 115 which is connected through a so-called “single-wire” interface 130 with an analog input port 122 of a microcontroller 120. The “single-wire” interface in fact uses two wires wherein the second wire is generally coupled to ground as a reference potential. In many environments such as, for example, vehicles, the ground connection is provided by the chassis and therefore may require only a very short wire or no wire at all if the sensor is mounted to the chassis. Thus, only the wire connected to pin 122 carries the actual analog information.
Such a sensor device also consumes power in the range of 2-3 mA which can be detrimental in certain low power applications. Moreover, the microcontroller must either provide for the necessary power supply or external circuitry must be provided to supply the thermistor with power. For example, a constant current source coupled with an analog input port or external circuitry that provides the necessary signals must be present as will be discussed in more detail below with respect to integrated active thermistors.
Unlike resistive sensors such as thermistors, there also exist linear active thermistor devices in the form of an integrated circuit device which do not require an additional signal-conditioning circuit and only consume current in the range of about 0.1 mA. The biasing circuit development overhead as mentioned above for thermistor solutions can be avoided by such a device. FIG. 1B shows the basic two measurement principles used in such an integrated device. On the left side there is shown a device that provides for current output wherein the temperature proportional current can be in the range of, for example, 1 μA/K. The equivalent circuit on the right side shows a solution for a voltage sensor that produces for example a temperature proportional voltage of 10 mV/K. The additional compensation circuit for providing a linearly proportional output signal is not shown in FIG. 1B. However, as can be seen in FIG. 1B, such devices require at least another pin for the power supply V.